Well, my personal opinion is that you should know how to run a code as a CA1/PGY-2.
It depends on who shows up, though, based on your institution. If you don't know, I'd ask.
At my institution, I often show up to find 20 people in the room, half of whom are standing around looking clueless, 1/4 are trying to find stuff and running around like a chicken with their head cut off, and the other 1/4 are often the "medicine" staff who are deeply engrossed in a chart biopsy.
Usually, I arrive, go to the head of the bed, stick the tube in, and then start telling people to actually do things. If an attending shows up sooner or later, they usually don't interfere.
So far, this system has seemed to work pretty good for me individually. The people that are "saveable" have been saved, and those who aren't haven't been. Either way, I'm not the one who made them "code", so I don't sweat it too much when I get there. I just do what needs to be done. And, try to keep composure among others in the room.
If you appear to be in charge, and you keep your cool, it's amazing how smoothly codes can go. If you lose your cool, it can be a disaster. That's the key thing to know about showing up at codes. If the room is out of control, take control. If everything is going smoothly, just fit in and ask how you can help while working to keep things flowing.
-copro